Rate initialization and overdrive pacing for capture threshold testing

ABSTRACT

Approaches for rate initialization and overdrive pacing used during capture threshold testing are described. Cardiac cycles are detected and the cardiac events of a cardiac chamber that occur during the cardiac cycles are monitored. The number of intrinsic beats in the cardiac events is counted. Initialization for a capture threshold test involves maintaining a pre-test pacing rate for the capture threshold test if the number of intrinsic beats in the cardiac events is less than a threshold. The pacing rate is increased for the capture threshold test if the number of intrinsic beats in the cardiac events is greater than the threshold.

RELATED PATENT DOCUMENTS

This application is a continuation of U.S. application Ser. No.13/306,615, filed on Nov. 29, 2011, which claims the benefit ofProvisional Patent Application Ser. No. 61/426,751, filed on Dec. 23,2010, to which priority is claimed pursuant to 35 U.S.C. §119(e) andwhich is hereby incorporated herein by reference.

FIELD

The present disclosure relates to implantable cardiac devices andmethods of operating cardiac devices.

SUMMARY

Some of the embodiments described in this disclosure relate to methodsused for capture threshold testing. According to some methods, cardiaccycles are detected and cardiac events of a cardiac chamber that occurduring the cardiac cycles are monitored. The number of intrinsic beatsin the cardiac events is counted. The pacing rate for a capturethreshold test is initialized. A pre-test pacing rate is maintained forthe capture threshold test if the number of intrinsic beats in thecardiac events is less than a threshold. The pacing rate is increasedfor the capture threshold test if the number of intrinsic beats in thecardiac events is greater than the threshold. The capture threshold testis performed using the initialized pacing rate.

In some implementations, if the pre-test rate is greater than a maximumrate, the rate initialization process is terminated.

Monitoring the cardiac events may include monitoring the cardiac eventsthat occur during the cardiac cycles until a predetermined number ofcardiac events have occurred.

Some implementations further include counting a number of consecutiveatrial events that are intrinsic beats or fusion beats to determine anintrinsic/fusion beat count.

According to some aspects, after initializing the atrial pacing rate,the pacing rate during the capture threshold test is set by maintaininga previous pacing rate if the intrinsic/fusion beat count is less than apredetermined value; and increasing the previous pacing rate if theintrinsic/fusion beat count is greater than the predetermined value.Detecting cardiac cycles may be performed by detecting certain endevents that signify the end of a cardiac cycle.

In some cases, the cardiac events are atrial events, the intrinsic beatsare atrial intrinsic, and the pacing rate is an atrial pacing rate.

In some cases, the cardiac events are ventricular events, the intrinsicbeats are ventricular intrinsic beats, and the pacing rate is aventricular pacing rate.

Some embodiments discussed in this disclosure involve cardiac pacingdevices. The device includes sensing circuitry configured to detectcardiac signals that include indications of cardiac events. A controlprocessor is configured to monitor the cardiac events that occur duringcardiac cycles and to count a number intrinsic beats in the cardiacevents. The control processor includes circuitry configured toinitialize a pacing rate for a capture threshold test (CTT). The controlprocessor is configured to maintain a pre-test pacing rate during thecapture threshold test if the number of intrinsic beats in the cardiacevents is less than a threshold and to increase the pre-test pacing rateduring the capture threshold test if the number of intrinsic beats inthe cardiac events is greater than the threshold. The cardiac deviceincludes pacing circuitry configured to deliver pacing during thecapture threshold test using the initialized pacing rate.

In some cases, the control processor is configured to determine if thepre-test rate is greater than a maximum rate and to end theinitialization if the pre-test rate is greater than the maximum rate.

According to some implementations, the control processor is configuredto monitor the cardiac events that occur during the cardiac cycles untila number of cardiac events have occurred.

The control processor may be configured to count a number of consecutivecardiac events that are intrinsic beats or fusion beats to determine anintrinsic/fusion beat count. The previous pacing rate may be maintainedif the intrinsic/fusion beat count is less than a predetermined valueand the previous pacing rate may be increased if the intrinsic/fusionbeat count is greater than the predetermined value.

In some cases, the cardiac events are atrial events, the intrinsic beatsare atrial intrinsic beats, and the pacing rate is an atrial pacingrate.

In some cases, the cardiac events are ventricular events, the intrinsicbeats are ventricular intrinsic beats, and the pacing rate is aventricular pacing rate.

Some embodiments include a control processor configured to control theperformance of a capture threshold test (CTT), the control processor isconfigured to count, during the CTT, a number of consecutive cardiacevents that are intrinsic beats or fusion beats to determine anintrinsic/fusion beat count, the control processor further configured tomaintain a previous pacing rate during the CTT if the intrinsic/fusionbeat count is less than a predetermined value and to increase theprevious pacing rate during the CTT if the intrinsic/fusion beat countis greater than the predetermined value.

The control processor may be configured to terminate the test if thepaced cardiac rate is greater than a predetermined rate. The controlprocessor may terminate the capture threshold test if a predeterminednumber of rate increases have occurred.

The above summary is not intended to describe each embodiment or everyimplementation of the present invention. Advantages and attainments,together with a more complete understanding will become apparent andappreciated by referring to the following detailed description andclaims in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram that illustrates a method of operating acardiac device to initialize a pacing rate for atrial capture thresholdtesting in accordance with embodiments described herein;

FIGS. 2A and 2B provide a flow diagram of a method of operating acardiac device to initialize the atrial pacing rate prior to atrialcapture threshold testing;

FIG. 3 is a flow diagram illustrating a method for atrialintrinsic/fusion beat management that may be implemented during anatrial capture threshold test;

FIGS. 4A and 4B shows a more detailed flow diagram for intrinsic/fusionbeat management which may be implemented during an atrial capturethreshold test to reduce the likelihood of occurrence of intrinsic orfusion beats during the atrial capture threshold test;

FIG. 5 illustrates a process for performing an atrial pacing rateincrease after a rate increase has been requested during rateinitialization prior to an atrial capture threshold test or during theatrial capture threshold test;

FIG. 6 illustrates an implantable cardiac rhythm management (CRM) systemthat may be used in connection with pacing methods in accordance withembodiments described herein;

FIG. 7 is a block diagram of the circuitry of the implantable cardiacdevice according to embodiments described herein;

FIG. 8 is a flowchart illustrating a method for classifying the cardiacresponse to pacing that may be implemented by a CRM device in accordancewith embodiments described herein;

FIG. 9A is a graph illustrating the morphology of a cardiac signalsensed following an atrial pace;

FIG. 9B is a diagram illustrating regions within the peak timinginterval used in pacing response discrimination in accordance withembodiments described herein;

FIG. 10 is a flowchart illustrating step down capture threshold testingwith pacing response classification based on the regions depicted inFIG. 9B in accordance with embodiments described herein;

FIG. 11 is a diagram illustrating regions used in pacing responsediscrimination in accordance with embodiments described herein;

FIGS. 12A-12B illustrate a flowchart illustrating step down capturethreshold testing with pacing response classification based on theregions depicted in FIG. 11 in accordance with embodiments describedherein;

FIG. 13 is a flowchart that illustrates an approach implementable in aCRM system for retrograde conduction management and PMT management inaccordance with embodiments described herein;

FIG. 14 is a timing diagram illustrating a scenario where loss ofcapture may be erroneously detected due to retrograde conduction;

FIG. 15 is a timing diagram illustrating retrograde management inaccordance with embodiments described herein;

FIG. 16 is a timing diagram illustrating PMT caused by retrogradeconduction;

FIG. 17 is a timing diagram illustrating PMT management in accordancewith embodiments described herein; and

FIG. 18 is a flowchart illustrating retrograde management and PMTmanagement in accordance with embodiments described herein.

DESCRIPTION OF VARIOUS EMBODIMENTS

Systems and devices described in this disclosure may include one of thestructures described herein or any combination of structures described.Processes may include one of the functions described or combinations offunctions. It is intended that such devices, systems, and/or processesneed not include all of the structures and/or functions described, butmay be implemented to include selected structures and/or functions. Suchdevices, systems and/or processes may be implemented to provide avariety of therapeutic or diagnostic features.

After delivery of a pacing pulse to a heart chamber, various cardiacresponses to the pacing pulse are possible. In one scenario, the pacingpulse may generate a propagating wavefront of depolarization resultingin a contraction of the heart chamber. In this scenario, the pacingpulse is said to have captured the heart chamber. Capture of the heartchamber may occur if the pacing pulse has sufficient energy and isdelivered during a non-refractory period. If the pacing pulse does notproduce contraction of the chamber, the cardiac response is referred toas non-capture or loss of capture. Non-capture may occur, for example,if the pacing pulse energy is too low, and/or if the pacing pulse isdelivered during a refractory period of the cardiac tissue. Fusionoccurs when a depolarization initiated by a pace merges with anintrinsic depolarization.

Approaches for determining pacing response described herein rely onconsistency in the morphology of the cardiac signal sensed following apacing pulse to discriminate between noncapture, capture, and fusionresponses. One or more features of the sensed cardiac signal followingpacing, e.g., peak magnitude and/or peak timing, may be analyzed withrespect to feature thresholds and/or timing intervals to determine thepacing response.

By way of example, the processes described herein may be used in acapture threshold test (CTT) used to determine the optimal energy forpacing. Capture detection allows the cardiac pacing device to adjust theenergy level of pace pulses to correspond to the optimum energyexpenditure that reliably produces a contraction. Embodiments describedherein are directed to methods and systems for pacing responseclassification that distinguishes between capture, noncapture, andfusion. The pacing response classification processes described hereinare based on the use of timing windows and amplitude thresholds totranslate atrial response peak amplitude and timing information intocapture, noncapture and fusion response classification.

The approaches for pacing response classification are described hereinusing atrial chamber response classification as an example. However, theapproaches are equally applicable to any one or more cardiac chambers,e.g. left atrial chamber, right atrial chamber, left ventricularchamber, and/or right ventricular chamber. The examples based on atrialresponse detection discussed herein are extendable to other chambers aswell.

Noncapture of the atrium by an atrial pace may allow retrogradeconduction to occur when a depolarization wave initiated in a ventricleby a pacing pulse or intrinsic activation of the ventricle travels backto the atrium producing a retrograde P-wave. A pacing pulse delivered tothe atrium will not result in capture if the atrial tissue is refractorydue to a retrograde P-wave. Retrograde P-waves may inhibit accuratedetermination of the capture threshold during a capture threshold test.Further, retrograde conduction to the atrium may cause pacemakermediated tachycardia (PMT).

Those skilled in the art will appreciate that reference to a CTTindicates a method of determining the capture threshold in one or moreof the left atrium, right atrium, left ventricle, and right ventricle.In such a procedure, the pacemaker, automatically or upon command,initiates a search for the capture threshold of the selected heartchamber. The capture threshold is defined as the lowest pacing energythat consistently captures the heart. Other procedures for implementinga CTT may be utilized. In one example, the pacing energy may beincreased in discrete steps until capture is detected. In anotherexample, the pacing energy may be adjusted according to a binomialsearch pattern, or other search patterns.

In one example of an automatic CTT, the pacemaker delivers a sequence ofpacing pulses to the heart and detects the cardiac pacing responses tothe pace pulses. The energy of the pacing pulses may be decreased indiscrete steps until a predetermined number of noncapture responsesoccur. The pacemaker may increase the stimulation energy in discretesteps until a predetermined number of capture responses occur to confirmthe capture threshold. An atrial CTT may be performed using the pacingresponse classification, atrial sense and fusion management, retrogrademanagement, and/or PMT management methods of embodiments describedherein.

Capture threshold determination is distinguishable from automaticcapture detection, a procedure that typically occurs during therapeuticpacing, rather than during test mode pacing. Automatic capture detectionverifies that a delivered pace pulse results in a captured response.When a captured response is not detected following a pace pulse, thepacemaker may deliver a back up safety pace to ensure consistent pacing.If back up pacing is implemented, the back up pace may be delivered, forexample, about 70-80 ms after the initial pace pulse. If a predeterminednumber of pace pulses delivered during normal pacing do not produce acaptured response, the pacemaker may initiate a CTT to determine thecapture threshold. Alternatively, if a predetermined number of pacingpulses do not produce a captured response, the pacemaker may adjust thepacing energy for the next pacing pulse.

FIG. 1 is a flow diagram that illustrates a method 100 of operating acardiac device to initialize a pacing rate for an atrial CTT inaccordance with embodiments described herein. The method 100 determinesif the current pacing rate is at a rate high enough to avoid fusionduring the atrial CTT. Prior to the commencement of the atrial CTT, thecardiac device monitors 110 atrial events that occur during apredetermined number of cardiac cycles. The atrial events may be pacedatrial beats, intrinsic atrial beats, and/or fusion atrial beats, forexample. In some cases, the device looks for a minimum number of theatrial events, e.g., about ten atrial events that occur during aboutfifteen cardiac cycles. If the minimum of atrial events is not detectedduring the cardiac cycles, then the atrial CTT may be terminated.Optionally, a failure code indicating the reason for the failure of theatrial CTT may be generated.

The number of the atrial events that are intrinsic atrial beats arecounted 130. The device compares 140 the number of intrinsic atrialbeats that occur during monitoring the atrial events to a threshold. Insome implementations, the threshold is about six, so if the number ofsensed intrinsic atrial beats is greater than or equal to about sixintrinsic beats in about ten atrial events, there is sufficientintrinsic atrial activity to indicate that fusion is likely to occurduring the atrial CTT. The cardiac device increases 150 the pacing rateto avoid fusion. For example, the device may increase the pacing rate byabout ten beats per minute. If the number of sensed intrinsic atrialbeats that occur during monitoring 110 the atrial events is less thanthe threshold, it is likely that the pacing rate is adequate to avoidfusion. The cardiac device maintains 160 the current pacing rate.

FIGS. 2A and 2B illustrate a more detailed flow diagram of a method ofoperating a cardiac device to initialize the atrial pacing rate prior toan atrial CTT. The device counts the number of consecutive cardiaccycles, counts the number of atrial events that occur during theconsecutive cardiac cycles, and counts the number atrial events that arefusion or intrinsic events beats.

The device monitors cardiac cycles which are terminated by cardiac cycleend events. Cardiac cycle end events for various pacing modes areprovided in Table 1.

TABLE 1 Cardiac Cycle End Events DDI(R) or DDI(R) or DDD(R) (LV only)DDD(R) (Bi-V or AAI(R) RV sense RV sense A sense (non- refractory) LVpace RV pace A pace LV pace noise RV pace noise A pace noise inhibitedinhibited inhibited LV pace LVPP inhibited

Where LV/RV/A pace noise inhibited is a pace that is scheduled, butinhibited due to the noise response of the device; LV pace LVPPinhibited is a pace that is inhibited if it would occur within apredetermined interval following a left ventricular depolarization. If acardiac end event occurs 210, a consecutive cycle counter is incremented220. The device compares 230 the number of consecutive cardiac cycles toa cycle count threshold. The cycle count threshold may be about 15 or20, for example. If the number of consecutive cardiac cycles is greaterthan the cycle threshold without detecting enough atrial events, rateinitialization for the atrial CTT may be terminated and/or retried 235.If, however, the number of consecutive cardiac cycles is less than thecycle count threshold, the method checks 240 to determine if the cardiacevent was a non-refractory atrial event, i.e., an atrial fusion beat,non-refractory intrinsic atrial beat, or an atrial pace. Refractoryatrial sensed beats are not included in the rate initialization processin this example.

If the cardiac event is not 240 an atrial event, the process waits 265for the next event. If the cardiac event is 240 an atrial event, theatrial event counter is incremented 245. If the cardiac event is anatrial event and the atrial event is 250 an intrinsic atrial beat, theatrial intrinsic beat counter is incremented 255. If the atrial event isnot an atrial intrinsic beat then the process waits 265 for the nextevent.

The device checks 260 to determine if the number of atrial events isgreater than or equal to the atrial event threshold. The atrial eventthreshold may be about ten atrial events, for example. If the devicedetermines 260 that the number of atrial events is less than the atrialevent threshold, there are not enough atrial events to assess and thedevice waits 265 for another atrial event. The method continues to loopuntil the number of cardiac cycles exceeds the cycle threshold or thenumber of atrial events is greater than or equal 260 to the atrial eventthreshold. If the number of atrial events is equal to or greater than260 the atrial event threshold, then a sufficient number of atrialevents have occurred and the rate initialization process continues (seeFIG. 2B).

The device determines the pre-test cardiac rate (or cardiac cycleinterval) and checks to determine if the pacing rate can be increasedfor the CTT. If the pre-test cardiac rate is 270 above a maximum rate,e.g., about 110 beats per minute, then the atrial CTT is terminatedand/or retried 275 at a later time. If, however, the pre-test cardiacrate is below or equal to 270 the maximum cardiac rate, the deviceproceeds with comparing 280 the number of counted atrial intrinsic beats(counted by the atrial intrinsic beat counter) to the atrial intrinsicbeat threshold. In some cases, the atrial intrinsic beat threshold maybe equal to about four or five, for example. If the number of atrialintrinsic beats is greater than the intrinsic beat threshold, the atrialpacing rate is too low to avoid atrial intrinsic and/or fusion beatsduring the atrial CTT. A rate increase is requested 285 and the rateinitialization process is completed 290. If the number of atrialintrinsic beats is less than or equal to the intrinsic beat threshold,the atrial pacing rate is high enough to provide adequate pacing duringthe atrial CTT. The rate initialization process does not request 285 arate increase and the current pacing rate is maintained. In someimplementations, instead of keeping track of the number of intrinsicatrial beats in the atrial events, the device may instead keep track ofthe number of paces delivered and compare the number of paces deliveredto a paced beat threshold.

Pacing rate initialization as discussed above is implemented to decreasethe likelihood of fusion beats during an atrial CTT. Alternatively oradditionally, the pacing rate may be adjusted during the atrial CTT iftoo many atrial sensed and/or atrial fusion beats occur during the CTT.An atrial intrinsic/fusion counter (I/F counter) keeps track ofconsecutive atrial beats that are either intrinsic beats or fusionbeats. The I/F counter is reset if an atrial pace occurs. If more than apredetermined number of atrial intrinsic or fusion beats (e.g., 5 atrialintrinsic or fusion beats of which at least 2 are atrial intrinsicbeats) occur at any time after rate initialization, the device willperform a rate increase. Each rate increase increases the pacing rate byabout 10 beats per minute (bpm) with a maximum number of rate increasesin any CTT, e.g., about 2 rate increases, and a pacing rate upper limit,e.g., about 110 bpm. Rate increases may be applied to the filteredaverage rate rather than the lower rate limit (LRL), i.e., the rate isincreased 10 beats per minute (bpm) above the current average ratherthan 10 bpm above the last pacing rate or the LRL.

FIG. 3 is a flow diagram illustrating a method 300 for atrialintrinsic/fusion beat management that may be implemented during anatrial CTT. During the CTT, atrial cardiac events are monitored 320. Forexample, the monitored atrial events can include intrinsic atrial beats,paced atrial beats, and atrial fusion beats. The device counts 330 thenumber of consecutive intrinsic atrial beats or atrial fusion beats. Theintrinsic atrial beats are intrinsic beats that occur before an atrialpacing pulse is delivered for the cardiac cycle and these intrinsicbeats may inhibit delivery of the scheduled pacing pulse. The deviceresets the counter when a paced atrial beat occurs. If the number ofconsecutive intrinsic atrial beats or atrial fusion beats is greaterthan an intrinsic/fusion threshold (I/F threshold), with a minimumnumber of intrinsic beats, the pacing rate is increased 350. If thenumber of consecutive intrinsic beats or fusion beats does not reach theI/F threshold, the current pacing rate is maintained 360.

FIG. 4 shows a more detailed flow diagram for intrinsic/fusion beatmanagement which may be implemented during an atrial CTT to reduce thelikelihood of occurrence of intrinsic or fusion beats during the CTT.After the occurrence of a cardiac event, the device determines 405 ifthe cardiac event is an event that ends a cardiac cycle. Cardiac cycleend events for various pacing modes are provided in Table I. If thecardiac event is a cardiac cycle end event, the device updates 407 thecardiac cycle time using the interval ended by the cardiac cycle endevent. The cardiac cycle time is determined as a weighted average ofcardiac cycle intervals. It will be appreciated that the cardiac rate isthe inverse of the cardiac cycle time and that either parameter may beused as an indication of the rate of occurrence of cardiac events. Inone implementation, the weighted average used to update the cardiaccycle time (CT) is calculated as Equation 1.

CT=A1*current value of CT+A2*previous value of CT  [1]

where A1 and A2 can be any appropriate coefficients, e.g. A1=0.25 andA2=0.75.

The CT may be smoothed using an infinite impulse response (IIR) filterimplementing Equation 1 to lessen the effects of spurious fast beats.

The device determines 410 whether or not the present cardiac rate isgreater than a maximum cardiac rate. The maximum cardiac rate may beabout 110 bpm, for example. If the present cardiac rate, i.e., the CTdetermined using Equation I, is not greater than a maximum rate, thedevice checks if the event is an atrial event 420 (FIG. 4B). If thepresent cardiac rate is greater than 410 a maximum cardiac rate, aconsecutive high rate cycle counter (CHR counter) is incremented and atotal high rate cycle counter (THR counter) is incremented. The CHRcounter counts the number of consecutive beats that have a CT greaterthan the maximum rate. The THR counter counts the total number of beats(consecutive or non-consecutive) that have a CT greater than the maximumrate. If the CHR counter exceeds a maximum number (maxCHR) or if the THRcounter exceeds a maximum number (maxTHR), then the threshold testterminates and may be attempted at a later time.

If neither CHR nor THR counters have reached their respective terminalcounts, the device determines 420 (FIG. 4B) that the event is an atrialevent. The device checks 422 if the atrial event is an atrial pace or anoise inhibited atrial pace. If the atrial event is not 422 an atrialpace or a noise inhibited atrial pace, the device determines 426 whetheror not the atrial event is an atrial refractory sensed event, i.e., anatrial event sensed during a refractory period, such as the postventricular atrial refractory period (PVARP). If the atrial event is 426a refractory sensed event, the next scheduled atrial pace enforces aminimum interval 427 so that the next scheduled atrial pace occurs afterthe atrial tissue recovers and is no longer refractory following therefractory sensed event. For example, the next scheduled atrial pace maybe delayed so that the next pace occurs at least about 300 ms after thesensed refractory atrial event.

If the atrial event is not 426, 428 a non-refractory intrinsic event,then the I/F counter is incremented 430. The I/F counter countsnon-refractory intrinsic atrial beats and atrial fusion beats. If theatrial event is a non-refractory intrinsic event, then the atrialintrinsic event (AlE) counter is incremented. The I/F counter is checked432 to determine if the I/F counter value is greater than an I/Fthreshold and the AlE counter is checked to determine if the AlE counteris greater than the AlE threshold. In some cases, the I/F threshold maybe about 5 and the AlE threshold may be about 2. If the I/F count doesnot exceed 432 the I/F threshold or the AlE counter does not exceed theAlE threshold, then the process loops to look for the next cardiacevent. If the number of intrinsic atrial beats and fusion beats countedby the I/F counter exceeds 432 the I/F threshold and the number of AlEbeats exceeds the AlE threshold, then a rate increase is requested 434.

At step 422, if the atrial event is a delivered atrial pace or an atrialpace that was inhibited by noise, the device determines 424 whether ornot the paced atrial rate is greater than a maximum atrial pacing rate.For example, the device determines if the atrial pacing ratecorresponding to the interval between atrial paces (Ap-Ap) or theinterval between an atrial pace and a noise inhibited atrial pace Ap-Ap(inhibited) is greater than the maximum pacing rate, e.g., about 110bpm. If the atrial pacing rate exceeds the maximum atrial pacing rate,the CTT is terminated. If the atrial pacing rate is less than themaximum atrial pacing rate and fusion is not present, then the I/F andAlE counters are cleared 436 and the process loops to check the nextcardiac event.

FIG. 5 illustrates a process for performing an atrial pacing rateincrease after a rate increase has been requested during rateinitialization prior to an atrial CTT (see, FIGS. 1, 2A and 2B) orduring the atrial CTT (see, FIGS. 3, 4A and 4B). If a rate increase hasbeen requested, the device determines 510 whether or not the number ofrate increases in the current test is greater than a maximum rateincreases threshold. For example, the maximum rate increases thresholdmay be equal to about two such that if there have already been two raterequest increases, then no additional rate increases are performed andthe CTT is terminated. If, however, the number of rate increases isbelow 510 the maximum rate increases threshold, the device determines520 if the current rate, e.g., the cardiac rate determined usingEquation 1 above, is greater than the maximum cardiac rate. The maximumcardiac rate may be about 110 bpm, for example. If the current rate isgreater than 520 the maximum cardiac rate, the test is terminated andmay be retried at a later time. If the current rate is less than 520 themaximum cardiac rate, the pacing rate is increased 530. For example, theatrial pacing rate may be increased by about 10 bpm above the weightedaverage value determined by Equation 1. The atrial pacing rate increaseis capped by the maximum pacing rate. For example, if the maximum pacingrate is 110 bpm, the pacing rate increment is 10 bpm, and the currentpacing rate is 105 bpm, then a pacing rate increase would only increasethe pacing rate to 110 bpm. The I/F, AlE counters are cleared 540 andthe number of rate increases counter is incremented. In addition, thecapture counter, non-capture counter and I/F counter may also becleared. The CTT proceeds 550 at the increased atrial rate. The timingwindows and respiration modulation index (RMI) used for the CTT may beinitialized or reinitialized after the rate increase.

Modulation of the cardiac response signal due to respiration causes theslight changes in the signal that may obstruct accurate cardiac responsedetermination. The CTT may determine a respiration modulation index(RMI) used to adjust thresholds and/or templates, etc., for cardiacresponse determination, e.g., capture, non-capture, fusion thresholds.After initializing the RMI, e.g., at the beginning of each test, the RMIis used throughout that test. RMI is a measure of respiration affect onatrial evoked response (AER) amplitude. During RMI determination, thedevice gathers AER peak amplitude data for a number of atrial pacesand/or for a maximum number of cardiac cycles. In some cases the devicegathers AER peak amplitude data for about 12 atrial paces and or about25 cardiac cycles. RMI may be calculated by using Equation [2].

RMI=[(Avg AER amplitude of X beats)−(Min AER amplitude of X beats)]/(AvgAER amplitude of X beats)  [2]

Successful RMI values may be between about 0 to 0.3, for example. Somevalues may still be determined to be successful if they do not fallwithin this specified range and an upper threshold may be invoked. Forexample, using the acceptable values 0 to 0.3, RMI values greater than0.3 but less than 0.4 may be acceptable, but RMI may be set to 0.3 forcapture detection threshold calculations. RMI values greater than orequal to 0.4 in this example, may signify either that fusion is present,giving wide variation in evoked response peak amplitudes, or that thealgorithm is pacing sub-threshold. In some cases, even when the upperthreshold limit is not met and/or when the RMI initialization does notcomplete in the maximum number of cardiac cycles, the device maycontinue with threshold tests if other criteria are met. For example,the criteria may include that one or more of the followingoccurrences: 1) there are less than a number e.g. 2, small evokedresponses (0.35 mV, for example); 2) there is a recent successfulthreshold test; 3) the capture threshold is below a value such as 2.5volts. Other criteria may also or alternatively be used. One or more ofthe criteria may be used to determine whether the device continues withthreshold tests. The reasoning behind the use of the criteria is thatRMI initialization may fail due to factors such as fusion orsub-threshold pacing. Fusion can be managed while sub-threshold pacingindicates a problem. Small evoked responses are often a sign ofsub-threshold pacing, so too many sub threshold paces are undesirable.Because capture thresholds generally do not change suddenly, e.g., from2.5 to 4 V within the “recent” test time of about 21 hours, it isassumed that if the previous threshold was <=2.5 V, the current RMIfailure is caused by something other than sub-threshold pacing at 4.0 V.If these criteria are met, it may reasonably be concluded that fusionwas the cause of the RMI failure and the test is allowed to continuewith a default RMI value.

Referring now to FIG. 6 of the drawings, there is shown a cardiac rhythmmanagement system that may be used to implement atrial capture thresholdtesting according to embodiments described herein. The system in FIG. 6includes an implantable cardiac device (ICD) 600 electrically andphysically coupled to a lead system 602. The header and/or housing ofthe ICD 600 may incorporate one or more electrodes 681 a, 681 b used toprovide electrical stimulation energy to the heart and to sense cardiacelectrical activity.

The lead system 602 is used to detect electric cardiac signals producedby the heart and to provide electrical energy to the heart under certainpredetermined conditions to treat cardiac arrhythmias. The lead system602 may include one or more electrodes used for pacing, sensing, and/orcardioversion/defibrillation. In the embodiment shown in FIG. 6, thelead system 602 includes an intracardiac right ventricular (RV) leadsystem, an intracardiac right atrial (RA) lead system, an intracardiacleft ventricular (LV)/left atrial (LA) lead system. The lead system 602of FIG. 6 illustrates one embodiment that may be used in connection withthe capture threshold testing methodologies described herein. Otherarrangements may additionally or alternatively be used.

The lead system 602 may include intracardiac leads implanted in a humanbody with portions of the intracardiac leads inserted into a heart. Theintracardiac leads include various electrodes positionable within theheart for sensing electrical activity of the heart and for deliveringelectrical stimulation energy to the heart, for example, pacing pulsesand/or defibrillation shocks to treat various arrhythmias of the heart.

The lead system may include one or more extracardiac leads havingelectrodes, e.g., epicardial electrodes, positioned at locations outsidethe heart for sensing and pacing one or more heart chambers.

The right ventricular lead system illustrated in FIG. 6 includes anSVC-coil 641, an RV-coil 642, an RV-ring electrode 663, and an RV-tipelectrode 653. The right ventricular lead system extends through theright atrium and into the right ventricle. In particular, the RV-tipelectrode 653, RV-ring electrode 663, and RV-coil electrode 642 arepositioned at appropriate locations within the right ventricle forsensing and delivering electrical stimulation pulses to the rightventricle. The SVC-coil 641 is positioned at an appropriate locationwithin the right atrium chamber or a major vein leading to the rightatrial chamber.

In one configuration, the RV-tip electrode 653 referenced to the canelectrode 681 b may be used to implement unipolar pacing and/or sensingin the right ventricle. Bipolar pacing and/or sensing in the rightventricle maybe implemented using the RV-tip and RV-ring electrodes 653,663. In yet another configuration, the RV-ring 663 electrode mayoptionally be omitted, and bipolar pacing and/or sensing may beaccomplished using the RV-tip electrode 653 and the RV-coil 642, forexample. The right ventricular lead system may be configured as anintegrated bipolar pace/shock lead. The RV-coil 642 and the SVC-coil 641can be used as defibrillation electrodes.

The left heart lead includes an LV distal electrode 655 and an LVproximal electrode 654 located at appropriate locations in or about theleft ventricle for sensing signals of the left ventricle and/ordelivering electrical stimulation to left ventricle. In the example ofFIG. 16, the left heart lead also includes optional left atrialelectrodes 656, 657. The left heart lead may be guided into the rightatrium via the superior vena cava. From the right atrium, the left heartlead may be deployed into the coronary sinus ostium and may be guidedthrough the coronary sinus to a coronary vein. This vein is used as anaccess pathway for leads to reach the surfaces of the left atrium and/orleft ventricle which are not directly accessible from the right side ofthe heart.

Unipolar pacing and/or sensing in the left ventricle may be implemented,for example, using the LV distal electrode 655 referenced to the canelectrode 681 b. The LV distal electrode 655 and the LV proximalelectrode 654 may be used together as bipolar sense and/or paceelectrodes for the left ventricle. The left heart lead and the rightheart leads, in conjunction with the ICD 600, may be used to providecardiac resynchronization therapy such that the ventricles and/or atriaof the heart are paced substantially simultaneously, or in phasedsequence, to provide enhanced cardiac pumping efficiency for patientssuffering from congestive heart failure.

The right atrial lead includes a RA-tip electrode 652 and an RA-ringelectrode 651 positioned at appropriate locations in the right atriumfor sensing and pacing the right atrium. In one configuration, theRA-tip 652 referenced to the can electrode 681 b, for example, may beused to provide unipolar pacing and/or sensing in the right atrium. Inthis configuration, RA-ring electrode 651 referenced to the canelectrode 681 b, for example, may be used to provide sensing of the RAevoked response. In another configuration, the RA-tip electrode 652 andthe RA-ring electrode 651 may be used to effect bipolar pacing and/orsensing.

FIG. 6 illustrates one embodiment of left atrial electrodes 656, 657.Unipolar pacing and/or sensing of the left atrium may be accomplished,for example, using the LA distal electrode 657 to the can 681 b pacingvector. The LA proximal 656 and LA distal 657 electrodes may be usedtogether to implement bipolar pacing and/or sensing of the left atrium.

Referring now to FIG. 7, there is shown a block diagram of an embodimentof a cardiac system 700 suitable for implementing atrial capturethreshold testing as described herein.

The cardiac system 700 includes a control processor 740 capable ofcontrolling the delivery of pacing pulses or defibrillation shocks tothe right ventricle, left ventricle, right atrium and/or left atrium.The pacing pulse generator 730 is configured to generate pacing pulsesfor treating bradyarrhythmia, for example, and/or for synchronizing thecontractions of contralateral heart chambers using biatrial and/orbiventricular pacing.

The control processor 740 may include an arrhythmia detector thatoperates to detect atrial or ventricular tachyarrhythmia orfibrillation. Under control of the control processor 740, thecardioversion/defibrillation pulse generator 735 is capable ofgenerating high energy shocks to terminate the detected tachyarrhythmiaepisodes.

The pacing pulses and/or defibrillation shocks are delivered viamultiple cardiac electrodes 705 electrically coupled to a heart anddisposed at multiple locations within, on, or about the heart. One ormore electrodes 705 may be disposed in, on, or about a heart chamber orat multiple sites of the heart chamber. The electrodes 705 are coupledto switch matrix 725 circuitry that is used to selectively couple theelectrodes 705 to the sense circuitry 710 and the therapy pulsegenerators 730, 735.

The cardiac system 700 includes a pacing response classification (PRC)processor 715. In some embodiments, the PRC processor 715 is configuredto discriminate between capture and non-capture. In some embodiments,the PRC processor is configured to discriminate between capture,noncapture and fusion. Pacing response classification is implemented bythe PRC processor 715 for capture threshold testing and/or captureverification during therapeutic pacing. The PRC processor 715 isconfigured to determine various thresholds and intervals useful in theanalysis of signals to determine the pacing response. For example, thePRC processor 715 may determine one or more of a pacing thresholdinterval (PTI), a pacing artifact threshold (PAT), and/or a capturedetection threshold (CDT). Discrimination between capture, noncapture,and fusion is performed by the PRC processor 715 based on comparison ofa cardiac signal sensed following a pacing pulse to one or more of theintervals or thresholds.

The control processor 740 includes a capture threshold module 743 thatcontrols the operation of the cardiac system during capture thresholdtesting. The control processor 740 may include an atrial refractoryperiod timer 741 for timing atrial refractory periods (ARP) and/or postventricular atrial refractory periods (PVARP) following atrial and/orventricular paces and/or senses. The control processor 740 mayoptionally include a retrograde management module 742 configured tocontrol pacing during retrograde management pacing cycles. The controlprocessor 740 may optionally include pacemaker mediated tachycardia(PMT) management module 744 configured to control pacing during PMTmanagement pacing cycles. The control processor 740 may also contain arate management module 771 and/or a respiration modulation index module772.

The capture threshold module 743 controls the delivery of paces by thepacing therapy pulse generator 730 during therapeutic pacing and duringcapture threshold testing. To determine the capture threshold, thecapture threshold module 743 may control the delivery of a sequence ofpacing pulses that incrementally step down or step up the pacing energyuntil a capture threshold is determined. Prior to beginning the capturethreshold test, the capture threshold module 743 may control pacingduring an initialization procedure. During the initialization procedure,the PRC processor 715 operates to determine thresholds and intervalsdescribed herein that are useful in cardiac pacing responseclassification. The thresholds and intervals determined in theinitiation procedure are then used to determine the pacing responses tothe threshold test paces. Prior to beginning the capture threshold test,the rate management module 771 may perform processes to initialize thecardiac rate for the CTT. The RMI module 772 may determine the RMI usedduring the CTT.

The CRM system 700 is typically powered by an electrochemical battery(not shown). A memory 745 stores data and program commands used toimplement the CTT approaches described herein along with other features.Data and program commands may be transferred between the CRM system 700and a patient-external device 755 via telemetry-based communicationscircuitry 750.

FIG. 7 shows a CRM system 700 divided into functional blocks. It isunderstood by those skilled in the art that there exist many possibleconfigurations in which these functional blocks can be arranged. Theexample depicted in FIG. 7 is one possible functional arrangement. Otherarrangements are also possible. For example, more, fewer or differentfunctional blocks may be used to describe a cardiac system suitable forimplementing the processes described herein. In addition, although theCRM system 700 depicted in FIG. 7 contemplates the use of a programmablemicroprocessor-based logic circuit, other circuit implementations may beutilized.

FIG. 8 is a flowchart illustrating a method for classifying the cardiacresponse to pacing a heart chamber, such as an atrial heart chamber,that may be implemented by a CRM device in accordance with embodimentsdescribed herein. The time between a delivered atrial pace and theevoked response signal peak is substantially consistent. A peak timinginterval (PTI) may be established for examining the cardiac signal todetermine the pacing response. The magnitude of the peak may be used toclassify the pacing response.

A method in accordance with one embodiment involves discriminatingbetween capture, noncapture, and fusion based on comparison of a sensedcardiac signal peak to a capture detection threshold (CDT), a pacingartifact threshold (PAT), and a peak timing interval (PTI). The PAT isdetermined 810 based on peak values of atrial signals of one or morenoncaptured cardiac cycles, e.g., about 2 to about 4 cardiac cycles. Thesignals used to determine the PAT may be sensed following sub-capturethreshold paces following a capture threshold test, for example. Thesensed atrial signals associated with noncapture are pacing artifactsignals that have a morphology exhibiting a pacing artifact without theevoked response morphology produced by capture. In variousimplementations, the PAT may be based on a combination of the peakvalues of the signals associated with noncapture. For example, the PATmay be based on the peak magnitude of a most recent cardiac signalassociated with capture, the largest one or more peak magnitudes of thesignals associated with noncapture, a median value of the magnitudes ofthe signals associated with noncapture, a mean value of the magnitudesof the signals associated with noncapture, a weighted average of themagnitudes of the signals associated with noncapture, or othercombination of the peak magnitudes of the signals associated withnoncapture. The PAT may include an offset to take into account thevariability of the peak magnitudes of the signals associated withnoncapture. In one example, the PAT is set to a percentage, such asabout 150%, of the peak magnitude of a most recent signal associatedwith noncapture.

A capture detection threshold (CDT) is determined 820 based on peakvalues of one or more evoked response signals, e.g., about 5 to about 10signals, detected during one or more captured cardiac cycles. Thesignals used to determine the CDT follow supra capture threshold paces.The signals associated with capture exhibit a morphology that includesan evoked response signal having a superimposed pacing artifact signal.Similarly to the PAT determination described above, the CDT may be basedon or a combination of the peak values of the signals associated withcapture. The CDT may be based on a most recent peak magnitude of asignal associated with capture, the largest one or more peak magnitudesof the signals associated with capture, a median value of the peakmagnitudes of the signals associated with capture, a mean value of thepeak magnitudes of the signals associated with capture, or a weightedaverage of the peak magnitudes of the signals associated with capture,or other combination of the peak magnitudes of the signals associatedwith capture. The CDT may include an offset to take into account thevariability of the peak magnitudes of the signals associated withcapture. The use of a weighted average for the CDT provides such anoffset, for example. In one embodiment, the CDT is set to a percentageof an average, e.g., about 70% of evoked response peak magnitudes.

A peak time interval (PTI) associated with an expected timing of theevoked response signal peak is used in conjunction with the PAT and theCDT. Discrimination between capture, noncapture, and fusion is based oncomparison of the magnitude of the cardiac signal peak relative to thePAT and CDT and comparison of the timing of the cardiac signal peakrelative to the PTI.

The PTI is determined based on the timing of peak values of one or moreevoked response signals detected during one or more captured cardiaccycles. The signals used to determine the PTI follow supra capturethreshold paces. The PTI may be determined based on the variability ofthe peak timing of the signals associated with capture, for example. Atypical value of the PTI is about 9 ms, for example. The PTI may bebased on a median value of the peak timings of the signals associatedwith capture, a mean value of the peak timings of the signals associatedwith capture, or a weighted average of the peak timings of the signalsassociated with capture, or other combination of the peak timings of thesignals associated with capture. The PTI may include predeterminedinterval offsets on either side of a most recent, average, mean, ormedian timing value, for example, where the interval offsets take intoaccount the variability of the peak timing of signals associated withcapture.

A cardiac signal following a pacing pulse of a cardiac cycle subsequentto the noncaptured cardiac cycles and the captured cardiac cycles issensed 830. A peak value of the sensed cardiac signal falling within thePTI is compared 840 to the PAT and to the CDT. The device discriminates850 between capture, noncapture, and fusion based on the comparison. Ifthe signal peak is less than the PAT, then the pacing response isdetermined to be noncapture. If the signal peak is greater than the CDT,then the pacing response is determined to be capture. If the signal peakfalls between the PAT and the CDT, then the pacing response may benoncapture or may be fusion.

FIG. 9A is a graph illustrating the morphology of a captured responsesignal 910 sensed following an atrial pace (Ap). The peak 911 of thecardiac signal 910 depicted in FIG. 9A has a magnitude (i.e., absolutevalue) larger than the PAT 930 and the CDT 940.

FIG. 9B is a diagram illustrating regions used in pacing responsediscrimination in accordance with one embodiment. FIG. 9B shows the PAT930, the CDT 940, and the PTI 950 which define regions 960, 970, and 980respectively associated with noncapture (NC), both noncapture andfusion, and capture. If the peak of a cardiac signal following pacingfalls within a particular region 960, 970, 980, then the cardiac pacingresponse is classified as likely to be the type of response or responsesassociated with the region 960, 970, 980.

In one implementation, a counter for a particular type of response isincremented each time a peak falls within a region associated with theparticular type of response. The counter increments may be integer orfractional increments. The counter increments may be based on thelikelihood that a particular type of pacing response has occurred. Forexample, region 970 is associated with both noncapture and fusion.However, it may be more likely that a peak falling in region 970 isfusion rather than noncapture. If a peak falls within region 970, thefusion counter (I/F counter) may be incremented by 1 and the noncapturecounter may be incremented by ½. In some scenarios, confirmation that aparticular pacing response has been occurring may require severalcardiac cycles. For example, confirmation of the particular type ofpacing response may occur if a counter for the particular type of pacingresponse reaches a predetermined value.

FIG. 10 is a flowchart illustrating step down capture threshold testingwith pacing response classification based on the regions depicted inFIG. 9B. The approaches described herein may advantageously be used inconnection with capture threshold testing. Prior to beginning the stepdown test, the CDT and/or PTI are initialized 1005 based on the peakmagnitudes of signals sensed following delivery of a series of supracapture threshold paces.

In one embodiment, the PAT is initialized to a predetermined value, suchas about 0.3 mV prior to the capture threshold test. The CDT and PTI areinitialized based on measured values of the peak magnitude and peaktiming of captured signals. Initialization of the CDT and PTI prior tothe test based on measured values provides patient specific values,enhancing the accuracy of capture testing. In addition, one or more ofthese parameters may be modified during and/or after the capturethreshold test based on most recent peak timing and peak magnitudevalues to further enhance the test accuracy.

The pacing energy and the pacing rate are initialized 1010 for the test.A pace is delivered 1015 and the cardiac signal following the pace issensed 1015. The peak magnitude (P_(M)) of the cardiac signal isdetermined 1020. If the peak magnitude is less than or equal to 1025 thePAT, then the pace did not capture the chamber and the noncapturecounter is incremented 1030. If the peak timing is within the PTI andthe peak magnitude is greater than the PAT but is less than or equal tothe CDT 1035, then the pacing response maybe noncapture or may befusion. Both the noncapture counter and the fusion counter areincremented 1040. If the peak timing is within the PTI and the peakmagnitude is greater than the CDT, then the pacing response is 1045capture and the capture counter is incremented 1050. Otherwise, theresponse is determined 1046 to be fusion.

The amounts that the counters for each type of response are incrementedmay be integer or fractional amounts. In some implementations, theamount that a particular counter is incremented is associated with thelikelihood that the type of pacing response occurred. For example, ifthe peak magnitude falls between the PAT and the CDT, fusion is morelikely than noncapture. In this scenario, the fusion counter may beincremented by 1 and the noncapture counter incremented by ½.

If the noncapture counter reaches 1055, 1056 a predetermined value,e.g., about 2, for paces having the same energy, then loss of capture isconfirmed and the capture threshold is determined 1060. If the fusioncounter reaches 1065 a predetermined value, e.g., about 5 intrinsicbeats or fusion beats, and at a least a minimum number of atrialintrinsic beats have occurred, e.g., about 2, then the pacing rate isincreased 1070 to decrease the occurrence of intrinsic/fusion beats. Ifthe capture counter reaches 1075 a predetermined value, e.g., about 3for paces having the same energy, then the pacing energy is stepped down1080 and the test continues until the capture threshold is determined1060.

In some implementations, the PAT, CDT, and PTI may be initialized beforethe test and/or one or more of these parameters may be modified duringthe test, such as during every cardiac cycle, and/or may be modifiedafter the test. The PAT may be re-initialized in the case of certainfailures.

In one example, the peak timing and/or peak magnitude may be determinedfor the cardiac signal of each beat. The peak timing and/or peakmagnitude may be combined with peak timings and magnitudes of one ormore previous beats to dynamically modify the PTI and CDT during thetest. Modifying the PTI and/or CDT during the test may be used to adaptto changing patient conditions, providing more accurate values for theseparameters. The PAT may be modified after the test based on one or morenoncaptured signals, detected after the capture threshold is determined.Modifying the PAT based on a particular patient's pacing artifactmorphology allows for adaptation to changing patient conditions overtime and provides more accurate pacing response classification.

FIG. 11 is a diagram illustrating regions used in pacing responsediscrimination in accordance with another embodiment. Fusion beatsusually exhibit large variations in peak timing of the cardiac signalwhen compared to captured beats. Regions corresponding to time intervalsbefore and/or after the PTI may be used for fusion discrimination. FIG.11 shows the PAT 1130, the CDT 1140, and the PTI 1150 which defineregions 1161-1163 associated with noncapture, region 1170 associatedwith noncapture and fusion, region 1180 associated with capture, andregions 1191-1194 associated with fusion. If the peak of a cardiacsignal following pacing falls within a particular region, then thecardiac pacing response is likely to be the type of response associatedwith the region.

As previously described, a counter for a particular type of response maybe incremented each time a peak falls within a region associated withthe particular type of response. The increments may be integer orfractional increments. The counter increments may be based on thelikelihood that a particular type of pacing response has occurred. Forexample, region 1170 is associated with both noncapture and fusion.However, it may be more likely that a peak falling in region 1170 isfusion rather than noncapture. If a peak falls within region 1170, thefusion counter may be incremented by 1 and the noncapture may beincremented by ½. In some scenarios, confirmation that a particularpacing response has been occurring may require several cardiac cycles.For example, confirmation of the particular type of pacing response mayoccur if a counter for the particular type of pacing response reaches apredetermined value.

FIGS. 12A-12B illustrate a flowchart illustrating step down capturethreshold testing with pacing response classification based on theregions depicted in FIG. 11. Prior to beginning the step down test, thePAT, CDT, and PTI are initialized 1205. The PAT is initialized to apredetermined value. The CDT is initialized based peak magnitudes,P_(m), of signals sensed following delivery of supra capture thresholdpaces. PTI is initialized based on the peak timing, P_(t), of signalssensed following delivery of supra capture threshold paces. The pacingenergy and the pacing rate are initialized 1210 for the test.

A pace is delivered 1215 and the cardiac signal following the pace issensed 1215. The peak magnitude, P_(m), of the cardiac signal isdetermined 1220. If the peak magnitude is less than or equal to 1225 thePAT, then the pace did not capture the chamber and the noncapturecounter is incremented 1230. If the peak magnitude is greater than thePAT and the timing of the peak does not fall 1235 within the PTI, thenthe pacing response is likely to be fusion and the fusion counter isincremented 1240. If the timing of the peak falls within the PTI and thepeak magnitude is greater than the PAT and less than or equal to 1245the CDT, then the pacing response may be fusion or noncapture. Thefusion counter and the noncapture counter are incremented 1250. If thepeak magnitude is greater than the CDT, then the pacing response is 1255capture and the capture counter is incremented 1260.

As previously described, the amounts that the counters for each type ofresponse are incremented may be integer or fractional amounts. In someimplementations, the amount that a particular counter is incremented isassociated with the likelihood that the type of pacing responseoccurred. For example, if the peak magnitude falls between the PAT andthe CDT, fusion is more likely than noncapture. In this scenario, thefusion counter may be incremented by 1 and the noncapture counterincremented by ½.

If the noncapture counter reaches 1270, 1271 a predetermined value,e.g., about 2, for paces having the same energy, then loss of capture isconfirmed and the capture threshold is determined 1275. If the fusioncounter reaches 1280 a predetermined value, e.g., about 5 intrinsic orfusion beats, with a minimum number of intrinsic beats, e.g., about 2,then the pacing rate is increased 1285 to avoid the occurrence ofintrinsic/fusion beats. If the capture counter reaches 1290 apredetermined value, e.g., about 3 for paces having the same energy,then the pacing energy is stepped down 1295 and the test continues untilthe capture threshold is determined 1275. Following the capturethreshold test, the PAT maybe updated based on the peak magnitude of oneor more noncaptured signals.

During pacing, if noncapture occurs, retrograde conduction from anintrinsic or paced ventricular depolarization may cause a falsenoncapture detection on the next pacing cycle. Retrograde conductionduring capture threshold testing, for example, may lead to erroneouscapture threshold determination. Retrograde conduction may also causeundesirable fast pacing, denoted pacemaker mediated tachyarrhythmia(PMT). Some embodiments described herein include methods and systemsthat provide for management of retrograde conduction and PMT.

The flowchart of FIG. 13 illustrates an approach implementable in a CRMsystem for retrograde conduction management and PMT management inaccordance with embodiments described herein. An atrial pace and aventricular pace are delivered 1310 during a cardiac cycle. A postventricular atrial refractory period (PVARP) is timed 1320 following theventricular pace. The CRM system determines 1330 if the atrial pacecaptured the atrium. In some embodiments, capture may be detected basedon comparison of peak magnitude and timing of the cardiac signalfollowing pacing to the CDT, PAT, and PTI as described above. In otherembodiments, capture may be determined using other capture detectionmethods known in the art.

If capture occurs, the depolarization associated with capture causestissue refractoriness, making retrograde conduction unlikely. Ifnoncapture occurs, the atrial tissue is not refractory after the paceand the ventricular depolarization may conduct retrogradely to theatrium. The system senses 1340 an atrial depolarization following thepacing cycle indicative of retrograde conduction. Retrograde managementis initiated 1350 if the atrial pacing pulse did not capture and anatrial depolarization is sensed during the PVARP. PMT management isinitiated 1360 if the atrial pacing pulse did not capture and an atrialdepolarization is sensed after the PVARP.

The timing diagram of FIG. 14 illustrates a scenario where noncapture iserroneously detected due to retrograde conduction. A noncaptured atrialpace 1410 and a ventricular pace 1430 are delivered during a firstcardiac cycle. A PVARP 1420 is timed is following the ventricular pace1430. In this cycle, the atrial pace may be accurately detected asnoncapture. However, confirmation of the loss of capture during acapture threshold test typically requires more than one noncapturedpace, such as several noncaptured paces detected consecutively or withina short period of time. If a noncapture event was caused by transienteffects, such as noise, rather than by the decrease in the pacingenergy, then loss of capture would not be confirmed because subsequentpaces would be captured and the test would continue. However, a patternof retrograde conduction may be initiated by the noncaptured pace,causing a single noncaptured pace to result in an erroneous loss ofcapture confirmation as described below.

Because the atrial pace 1410 did not produce capture, the depolarizationcaused by the ventricular pace causes retrograde conduction to theatrium. The retrograde conduction produces an atrial depolarization 1440causing the atrial tissue to become refractory. The atrialdepolarization 1440 does not initiate a new pacing cycle because itoccurs during PVARP 1420. The atrial pace 1411 for the next cycle isdelivered during the tissue refractory period 1450. Because the atrialpace 1411 is delivered while the tissue is refractory, the pace 1411 isdetected as noncapture. During a capture threshold test, the noncapturedatrial pace causes a false detection of noncapture because thenoncapture is the result of tissue refractoriness following retrogradeconduction rather than the change in the pacing energy level. Noncaptureof the atrial pace 1411 during the second cardiac cycle again causesretrograde conduction, an atrial depolarization 1441, and tissuerefractoriness. The pattern of false noncapture detection and retrogradeconduction may continue resulting in a confirmation of loss of captureand an erroneous capture threshold measurement.

The timing diagram illustrated in FIG. 15 illustrates retrogrademanagement in accordance with embodiments described herein. The atrialpace 1510 of the first cardiac cycle is noncaptured. The ventricularpace 1520 of the first cardiac cycle causes retrograde conduction to theatrium. An atrial depolarization 1540 produced by the retrogradeconduction causes the atrial tissue to become refractory during a tissuerefractory period 1550. The atrial depolarization 1540 does not initiatea new pacing cycle because the atrial depolarization occurs during PVARP1520. The CRM system senses the atrial depolarization 1540 that occursduring the PVARP 1520. The next scheduled atrial pace 1511 for the cyclefollowing the retrograde conduction is delayed until after the tissuerefractory period 1550 ends. Typically the period 1550 of tissuerefractoriness lasts less than 300 ms after the depolarization 1540 issensed, for example. Therefore, the next scheduled atrial pace 1511 inthis example is delayed until about 300 ms following the atrialdepolarization 1540.

The delayed pace 1511 is correctly classified as a captured pace. Thethird cardiac cycle includes an atrial pace 1512 and a ventricular pace1532 that are delivered at the scheduled time.

FIGS. 14 and 15 above illustrate retrograde conduction when theretrograde atrial depolarization occurs during PVARP. In this scenario,the retrograde atrial depolarization does not initiate a new pacingcycle. Retrograde conduction producing atrial depolarizations that occurafter PVARP has expired may result in PMT. PMT caused by retrogradeconduction is illustrated in the timing diagram of FIG. 16. The firstcardiac cycle includes a noncaptured atrial pace 1610 and a capturedventricular pace 1630. PVARP 1620 is timed following the ventricularpace 1630, 1631 for each cycle. The noncaptured atrial pace 1610 in thefirst cycle allows the depolarization initiated by the capturedventricular pace 1630 of the first cycle to conduct retrogradely to theatrium. An atrial depolarization caused by the retrograde conductioncauses a nonrefractory atrial sense. Because the atrial sense 1641occurs after expiration of PVARP 1620 (i.e., is a nonrefractory sense),the CRM system initiates a pacing cycle in the second cardiac cyclewhich is abnormally fast. The pattern of fast ventricular paces andretrograde atrial depolarizations that occur after PVARP continues inthe third and fourth cycles. The pacing cycles of FIG. 16 illustratePMT.

The timing diagram illustrated in FIG. 17 illustrates PMT management inaccordance with embodiments described herein. The first pacing cycleincludes a noncaptured atrial pace 1710 and a captured ventricular pace1730. The noncaptured atrial pace 1710 allows the depolarization causedby the ventricular pace to be retrogradely conducted to the atrium. Theretrograde conduction occurs after PVARP for the cycle has expired. Thenonrefractory atrial sense 1740 caused by the retrograde conduction isused by the CRM system to initiate a pacing cycle. The next ventricularpace 1731 is fast.

The CRM system initiates PMT management following the noncaptured atrialpace 1710 in the first cycle and the nonrefractory atrial sense 1740initiating the second cycle. The PVARP 1721 for the pacing cyclefollowing the noncaptured pace 1710, which is the second cycleillustrated in FIG. 17, is extended to break the PMT pattern. The nextatrial sense 1741 occurs in the extended PVARP 1721 and does notinitiate a pacing cycle. The third cardiac cycle illustrated in FIG. 17is a normal cycle.

The flowchart of FIG. 18 illustrates retrograde conduction managementand PMT management in accordance with embodiments described herein. Anatrial pace and a ventricular pace are delivered 1810 during a pacingcycle. An atrial refractory period is timed 1820 for the pacing cycle. Arefractory or nonrefractory atrial depolarization is sensed 1830. If theatrial pace did not capture 1840 the atrium and the atrialdepolarization was sensed 1850 after expiration of the refractoryperiod, then PVARP is increased 1860 for one cardiac cycle. For example,the PVARP may be extended to about 500 ms. Extending the PVARP to 500 msfor one cardiac cycle breaks the PMT.

If the atrial pace did not capture 1840 the atrium and the atrialdepolarization was sensed 1850 during the refractory period, the systemchecks to determine 1870 if the time between the atrial depolarizationand the next scheduled atrial pace is greater than the tissue refractoryperiod (TRP). If not, the time for the next scheduled pace is extended1880 to avoid retrograde conduction in subsequent cardiac cycles. Forexample, the time for the next pace may be extended so that there isabout 300 ms between the refractory atrial sense and the next pace.

It is understood that the components and functionality depicted in thefigures and described herein can be implemented in hardware, software,or a combination of hardware and software. It is further understood thatthe components and functionality depicted as separate or discreteblocks/elements in the figures in general can be implemented incombination with other components and functionality, and that thedepiction of such components and functionality in individual or integralform is for purposes of clarity of explanation, and not of limitation.

Various modifications and additions can be made to the preferredembodiments discussed hereinabove without departing from the scope ofthe present invention. Accordingly, the scope of the present inventionshould not be limited by the particular embodiments described above, butshould be defined only by the claims set forth below and equivalentsthereof.

What is claimed is:
 1. A method of operating a cardiac device,comprising: delivering pacing pulses to one or more electrodes that arecoupleable to a cardiac chamber of a heart at a first pacing rate, andcounting a number of intrinsic and/or fusion beats over a plurality ofcardiac cycles; after the counting step, initializing a capturethreshold test (CTT) including setting a capture threshold test (CTT)pacing rate, wherein the capture threshold test (CTT) pacing rate is setto a second pacing rate that is higher than the first pacing rate if thenumber of counted intrinsic and/or fusion beats over the plurality ofcardiac cycles reaches a predetermined threshold; and executing thecapture threshold test (CTT) using the initialized capture thresholdtest (CTT) pacing rate.
 2. The method of claim 1, further comprisinglimiting the second pacing rate from going beyond a predeterminedmaximum pacing rate.
 3. The method of claim 1, wherein the plurality ofcardiac cycles is a predetermined number of cardiac cycles.
 4. Themethod of claim 1, wherein the plurality of cardiac cycles are aplurality of consecutive cardiac cycles.
 5. The method of claim 4,wherein the capture threshold test (CTT) pacing rate is set to the firstpacing rate if the number of counted intrinsic and/or fusion beats overthe plurality of cardiac cycles does not reach the predeterminedthreshold.
 6. The method of claim 1, further comprises detecting cardiaccycle end events.
 7. The method of claim 1 further comprises detectingcardiac events including atrial events, and wherein the countedintrinsic and/or fusion beats are intrinsic and/or fusion atrial beats,and the capture threshold test (CTT) pacing rate corresponds to anatrial pacing rate.
 8. The method of claim 1 further comprises detectingcardiac events including ventricular events, and wherein the countedintrinsic and/or fusion beats are intrinsic and/or fusion ventricularbeats, and the capture threshold test (CTT) pacing rate corresponds to aventricular pacing rate.
 9. The method of claim 1, wherein the countingstep counts the number of intrinsic beats over the plurality of cardiaccycles.
 10. The method of claim 1, wherein the counting step counts thenumber of fusion beats over the plurality of cardiac cycles.
 11. Themethod of claim 1, wherein the counting step counts the number ofintrinsic beats and fusion beats over the plurality of cardiac cycles.12. A cardiac device, comprising: pacing circuitry for delivering pacingpulses to a cardiac chamber; sensing circuitry configured to detectcardiac events; a controller operatively coupled to the pacing circuitryand the sensing circuitry; the controller configured to determine fromthe detected cardiac events a number of intrinsic and/or fusion beatsand count the number of intrinsic and/or fusion beats over a pluralityof cardiac cycles; the controller further configured to set a capturethreshold test (CTT) pacing rate for a capture threshold test (CTT),wherein the capture threshold test (CTT) pacing rate is set higher ifthe number of counted intrinsic and/or fusion beats reaches apredetermined threshold while limiting the capture threshold test (CTT)pacing rate from going beyond a predetermined maximum pacing rate; andthe controller further configured to initiate a capture threshold test(CTT) at the capture threshold test (CTT) pacing rate by deliveringpacing pulses to the cardiac chamber using the pacing circuitry.
 13. Thecardiac device of claim 12, wherein the plurality of cardiac cycles is apredetermined number of cardiac cycles.
 14. The cardiac device of claim12, wherein the plurality of cardiac cycles are a plurality ofconsecutive cardiac cycles.
 15. The cardiac device of claim 12, whereinthe number of counted intrinsic and/or fusion beats corresponds to thenumber of intrinsic and/or fusion beats that occur in consecutivecardiac cycles.
 16. The cardiac device of claim 12, wherein thecontroller is configured to maintain a previous pacing rate as thecapture threshold test (CTT) pacing rate if the number of countedintrinsic and/or fusion beats does not reach the predeterminedthreshold.
 17. A cardiac device, comprising: pacing circuitry fordelivering pacing pulses to a heart chamber; sensing circuitryconfigured to detect cardiac signals that include indications of cardiacevents during a plurality of cardiac cycles; and a controller coupled tothe pacing circuitry and the sensing circuitry, the controllerconfigured to count a number of intrinsic beats in the cardiac events,the controller further configured to set a capture threshold test (CTT)pacing rate for a capture threshold test (CTT), wherein the capturethreshold test (CTT) pacing rate is set at a first pacing rate if thenumber of intrinsic beats in the cardiac events does not go beyond apredetermined threshold and the capture threshold test (CTT) pacing rateis set higher at a second pacing rate if the number of intrinsic beatsin the cardiac events goes beyond the predetermined threshold; and thecontroller is further configured to initiate a capture threshold test(CTT) at the capture threshold test (CTT) pacing rate by deliveringpacing pulses to the heart chamber using the pacing circuitry.
 18. Thecardiac device of claim 17, wherein the controller limits the capturethreshold test (CTT) pacing rate from going beyond a predeterminedmaximum pacing rate.
 19. The cardiac device of claim 17, wherein thecontroller is configured to repeatedly count the number of intrinsicbeats in the cardiac events, and set the capture threshold test (CTT)pacing rate to higher pacing rate if the number of intrinsic beats inthe cardiac events goes beyond the predetermined threshold until thenumber of intrinsic beats in the cardiac events does not go beyond thepredetermined threshold or until a predetermined maximum pacing rate isreached.
 20. The cardiac device of claim 19, wherein after the number ofintrinsic beats in the cardiac events does not go beyond thepredetermined threshold, the controller is configured to initiate thecapture threshold test (CTT) at the capture threshold test (CTT) pacingrate.